Labour is under fresh pressure over its handling of racism and anti-Semitism in the party amid revelations its London mayoral candidate used a racial slur, and criticism from the Chief Rabbi.

Sadiq Khan was accused of being unfit to take over the capital's City Hall after footage emerged of him describing moderate Muslims of being "Uncle Toms".

Meanwhile, Chief Rabbi Ephraim Mirvis said Labour had a "severe" problem with anti-Semitism that would get worse if the party's inquiry into the issue was used as a "sticking plaster" to placate voters.

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Labour leader Jeremy Corbyn set up an independent investigation into anti-Semitism and other forms of racism within the party as the row over the handling of controversial comments by prominent figures continued to engulf Labour.

The Chief Rabbi wrote: "If this inquiry turns out to be no more than a sticking plaster, designed to placate and diffuse until after the elections this week, the problem will surely get worse and not better.

"Jeremy Corbyn has stated that his party 'will not tolerate anti-Semitism in any form', and I very much hope that this inquiry will deliver on that pledge and be followed by decisive action.

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"All political parties share in the responsibility to rid our society of anti-Semitism but we cannot achieve that objective with political posturing or empty promises of action never to be fulfilled."

Corbyn has insisted there is not a "huge problem" with anti-Semitism in Labour and the issue is limited to a "very small" number of people.

Shadow cabinet minister Diane Abbott said it was "a smear to say that the Labour Party has a problem with anti-Semitism" and Unite union leader Len McCluskey said Corbyn was the victim of "a cynical attempt to manipulate anti-Semitism for political aims" that was "got up by the right-wing press aided and abetted by Labour MPs".

Mirvis warned it would be a mistake to treat the problem as a "political attack".

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He wrote: "There are many people, from all sectors of our society, who are demanding more responsibility, particularly from our politicians, for stamping out racism and anti-Semitism. The Labour Party has a long and proud history of doing precisely that.

"Yet, comments from senior and long-standing members of the party, both Jewish and not, show just how severe the problem has now become.

"Everyone agrees that there must be no place for anti-Semitism in our politics and I welcome the inquiry recently announced by the party's leadership. And yet, I would sound an urgent note of caution.

"In recent days, we have heard anti-Semitism in the Labour Party described variously as 'a smear' and as 'mood music' being manipulated by political opponents of Jeremy Corbyn.

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"There has been nothing more disheartening in this story than the suggestion that this is more about politics than about substance. The worst of mistakes, in trying to address this problem would be to treat it as a political attack which requires a political solution."

As voters prepare to go to the polls on Thursday, Tories attacked Labour's London mayoral candidate for comments he made in 2009 to Press TV.

During a discussion about Muslim voters, Sadiq Khan said: "You can't just pick and choose who you speak to. You can't just talk to Uncle Toms."

Khan's team said he "regrets" using the phrase, used against black people to suggest they are subservient to whites.

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A spokesman said: "This was a bad choice of phrase and Sadiq regrets using it. As communities minister at the time, Sadiq was talking about the need to engage with all parts of the community to tackle extremism and radicalisation - as he has pledged to do as mayor."

Paul Scully, MP for Sutton and Cheam, told the Daily Mail: "Once again, Sadiq Khan has shown he doesn't have the judgment to be Mayor of London.

"He's deeply hypocritical on race issue when it suits his political purpose. Labour must show they won't put up with attitudes like this in the party."